Summary:
Xanthosine phosphorylase (XapA or PNP-II) is one of two purine nucleoside phosphorylases (PNPs) in E. coli. PNPs use orthophosphate to cleave the N-glycosidic bond of the β-(deoxy)ribonucleosides to produce α-(deoxy)ribose1-phosphate and the free purine base. The bases can be utilized as precursors in the synthesis of nucleotides in the purine salvage pathways, as well as a nitrogen source. The pentose-1-phosphate formed serves as a carbon source. The other PNP, PNP-I purine nucleoside phosphorylase, differs in substrate specificity.

XapA does not cleave or synthesize adenosine and deoxyadenosine but catalyzes the phosphorolysis of xanthosine, inosine and guanosine with comparable efficiency. Therefore, this enzyme has been variously referred to as xanthosine phosphorylase and inosine-guanosine phosphorylase [Koszalka88, Dandanell05]. XapA enables E. coli to grow on xanthosine as the sole source of carbon [HammerJespersen80, Seeger95]. The amino acid sequence of XapA resembles that of mammalian purine nucleoside phosphorylases and has no similarity with PNP-I [Dandanell05].

Recently, an investigation of the NAD+ salvage pathway showed that a ΔnadC pncA double mutant can grow slowly in M9 minimal medium containing nicotinamide (NAM). This implied the presence of a previously undefined activity that routes NAM into NAD+synthesis. An additional mutation in xapA further decreased growth of the ΔnadC pncA mutant on NAM, and purified XapA was shown to catalyze formation of nicotinamide riboside from nicotinamide at low efficiency [Dong14].

XapA predominantly exists as a hexamer with some evidence for co-existence of a trimeric from [Dandanell05, HammerJespersen80]. Crystal structures of XapA were solved in the presence of guanine and phosphate as well as xanthine and sulfate. XapA forms a hexamer in a dimer-of-trimers conformation in the crystal structure [Dandanell05]. A Y191L mutation in the active site resulted in no detectable activity of XapA against xanthosine, although it retained activity against inosine and guanosine with altered affinities and reduced maximal velocities, while an N239D mutation changed the substrate specificity to adenosine [Dandanell05].

A genetic engineering study was directed at inosine production using a xapA mutant. It was concluded that due to apparent regulatory features in E. coli, other engineered organisms show higher inosine production [Shimaoka06].

XapA from E. coli K-12 can also catalyze the degradation of the synthetic xenobiotic substrate 5-nitrobenzisoxazole by the Kemp elimination mechanism, which is not known to occur in biological systems. This property has been demonstrated in enzyme engineering studies [Khersonsky11].

UniProt: No detectable activity with xanthosine as substrate, but largely retains its activity against other substrates, namely inosine and guanosine, although with altered affinities, higher and lower respectively, and clearly reduced maximal velocities for both.